It is commonplace to perform osteosynthesis of the backbone by using fixing devices that are fastened to the sacrum. This makes it possible to perform reduction movements between the various fixed stages, which movements can be contractions, distractions, or indeed reversals of vertebrae that have slipped forwards relative to the backbone, where the latter condition is known as "spondylolisthesis".
Distraction is often performed for reduction of spondylolisthesis. Partial reduction is thus obtained by putting the fibers of the patient's ligament and disk apparatus back under tension. Reduction is completed by using equipment in accordance with the operating technique associated therewith. Except under exceptional circumstances, the operation is performed bilaterally. Throughout the description below, only one side of the treatment is described.
Most blocks or plates known in the prior art for fixing to the sacrum suffer from certain drawbacks. One of these drawbacks lies in that the orientation of the bone-fastening screws is fixed, which means that the device clearly lacks flexibility in use, given variations from one patient to another concerning the configuration of the sacrum and the neighborhood thereof.
Another drawback of known plates or blocks for fixing to the sacrum lies in the risk of the fastening screws becoming unscrewed, particularly under the effect of relative micromovements that occur between the equipment and the sacrum. This can result in failure of the fixing to the sacrum.
Also, backbone osteosynthesis can be performed by using equipment serving to fix various functional units of the spinal column, often in a very rigid manner. This gives rise to a sudden change in the distribution of forces, and thus to resulting stress and deformation states for the disks underlying and overlying the fixing. At some stage in the long term, this modification gives rise to degeneration of the disks. This condition is known as the "hinge syndrome".
Document EP-A-0 625 337 discloses a bone fixing device, in particular for fixing the sacrum for osteosynthesis of the backbone, the device comprising elongate link means receiving at least one bone-fastening screw, which screw passes through an orifice formed in the link means, in which the link means include in the bottom region thereof a bearing surface of essentially circular cross-section, in which the head of the screw has a surface that is essentially spherical for bearing against said bearing surface, and in which the link means include a first thread in the orifice, the device also including a plug having a second thread suitable for co-operating with the first thread and for coming into clamping contact with said screw head to hold it in a desired angular position. That device enables the inclination of the screws relative to an elongate link element to be varied, and it also enables the spacing between the screws to be varied.
Nevertheless, that known device is disadvantageous in that the elongate link means are constituted by a plurality of components, specifically an elongate member of cup-shape and a plurality of fixing blocks distributed along the elongate member, each receiving the head of a screw, and at the bottom, a zone corresponding to the elongate member.
This means that it is extremely fiddly for the surgeon to handle the device and put it into place, and in particular it is necessary to slide a series of fixing blocks along the elongate member, and to hold them in position thereon.